There are many types of crops that are subject to frost damage, including both orchard crops and crops such as peppers that are borne on low-growing plants. However, it is well known that citrus fruits are especially likely to be damaged by freezing temperatures and that methods and means for preventing frost damage to citrus crops are of great concern. The present invention will therefore be described in relation to citrus fruit groves, but this is by way of a particularly important example, inasmuch as the invention is useful with other tree-borne crops and with field crops.
Citrus grove frost damage prevention is concerned with protecting the fruit on the trees rather than the trees themselves. Citrus trees are not ordinarily damaged by frost, but the fruit is ruined for commercial purposes if its water content freezes.
Heretofore three general methods have been employed for preventing frost damage in citrus groves. In one of these, fairly intense heat was generated locally at each of numerous points distributed through the grove; in another the air in the grove was stirred up to mix cold air just above the ground with warmer air from a higher level; and in the third, water was sprayed into the air to warm it.
At one time the most commonly used device for preventing citrus grove frost damage was the smudge pot and smudge pots are still used to a substantial extent. Smudge pots, which are containers of burning oil, are spaced through a grove at regular intervals at about 20 feet (about 6 meters) so that there are about 100 of them in each acre of grove. When the temperature drops to a threatening level, the smudge pots are ignited in the hope that they will generate enough heat to prevent frost damage to the fruit.
Smudge pots--so called because they burn oil with a characteristic smokey flame--present a serious air pollution problem that has been tolerated only as an alternative to accepting major economic losses in citrus groves.
To the citrus grower, pollution is undoubtedly a matter of concern, but a more serious objection is the high cost of operating smudge pots. On average, smudging requires 50 gallons of oil per acre per hour. The average frost danger period lasts about three hours, and with fuel oil paced at around one dollar per gallon, the fuel cost of protecting an acre of crop from an average frost is about $150. In addition, one laborer is needed for servicing about four acres of smudge pots. In a severe season, with several frosts or a few very prolonged periods of frost, the cost of smudging may well be so high as to raise the question of whether it would not be just as well to abandon the crop.
The air stirring expedient most often used in citrus groves is the wind machine, consisting of a large propeller, driven by a powerful engine. It is intended to mix the warmer air from just above the trees with the cold layer of air that forms among them. To be effective, a wind machine must necessarily consume a substantial amount of fuel. Since one wind machine can stir up air in only a limited area, a grove of any substantial size has to have a number of wind machines, each representing a substantial investment that is idle most of the time. Helicopters have been employed as a substitute for wind machines, but operating a helicopter is by no means inexpensive, and it requires an especially skilled pilot because the helicopter has to be flown at treetop level to be effective. Of course air mixing cannot be successful in any case if the higher level air is not warm enough.
Water spraying, which has the advantage of being non-polluting, puts heat into the air. But it can only be effective if enough sprays are used, each putting out an adequate volume of water. Therefore, water spraying not only requires the presence of a plentiful water supply but involves a large investment in pumping equipment and demands a high energy expenditure for pumping the water.
In the light of the present invention, these prior expedients for preventing frost damage can now be seen as inefficient because they did not take into account the conditions that create the possibility of such damage. In regions where citrus fruits are grown, frost conditions usually develop when the air is calm, or nearly so. As the ground loses its heat, small areas cool more rapidly than the rest of the ground, and the air over such cold areas, no longer heated by upward radiation, cools correspondingly and settles through the trees, displacing warm air upwardly in adjacent areas. If the subsiding air is cold enough, and remains in contact with the fruit long enough, the juice freezes--at least in parts of the fruit--and the crop is damaged.
Smudging tends to create very localized hot spots near ground level and generates correspondingly narrow rising currents of heated air. Because the air in these updrafts is so much hotter than the surrounding air, they rise at a high velocity, and consequently little horizontal mixing takes place. The dense smoke from smudging tends to block upward radiation, and thereby prevents some loss of heat, but much of the oil is wasted in convective heating of the air above the trees. Because of the lack of horizontal mixing, there can still be crop damage at locations between smudge pots, especially if the atmosphere is unusually cold.
Air mixing and water spraying involve much larger capital investments than smudging and, in addition, demand high expenditures for energy.
A fact not heretofore taken into account in applying any of these expedients is that a citrus grove ordinarily has one or more trouble spots where frost usually develops first and is most severe, and it has other areas which are less likely to sustain crop damage. With smudging in particular, efficient employment of manpower seldom accords with efficient operation of the smudge pots. Ordinarily the trouble spots can not be identified, but even if they are known, it is not always feasible to light the smudge pots in the trouble spots first and then work out towards the relatively frost-immune areas. What this means is that a certain amount of fuel is wasted in combatting frost that has not developed and may not develop. The same considerations obviously apply to the use of wind machines, helicopters and water sprays.